The present invention relates to a car number plate (license plate) illuminating lamp which is constructed so that the number plate can be illuminated uniformly and brightly over the entire surface thereof.
Conventionally, a car number plate lamp is generally arranged as shown in FIGS. 1A and 1B. That is, the number plate lamp is arranged to illuminate the number plate from a position above the number plate. In these figures, reference numeral 1 designates a number plate attached to the surface of a car body 2 at a designated position, and 3 designates a number plate lamp mounted at a position on the surface of the car body 2 above the number plate 1 and projecting outwardly from the surface of the car body. The number plate lamp 3 is constituted primarily by a lamp body 5 made of plastics or the like having an opening at its lower surface, a light source 6 disposed within the lamp body 5, a reflector 7, a transparent lens 8 closing the opening in the lamp body 5, and related components. The reflector 7 is curved in the form of a paraboloid of revolution, with the light source 6 at the focal point thereof, so that a part of the light radiated from the light source 6 is reflected by the paraboloid to obtain parallel rays 10a and 10b directed toward the lens 8. Steps 9 are formed over the entire inner surface of the lens 8 so that the parallel rays 10a, 10b, . . . reflected by the reflector 7 are refracted toward the number plate 1.
The steps 9 of the lens 8 are constituted by straight ridges extending parallel to one another in the left-and-right direction (forward and backward direction), As a result, the refractive angle of the light radiated from the light source 6 and transmitted through the lens 8 is small. Accordingly, light 11 rays advancing slantingly leftward and downward as well as rightward and downward are ineffective in illuminating the number plate 1 and are wasted. Therefore, this structure suffers from ineffective use of light, making bright illumination difficult.
In order to make the irradiation of the entire surface of the number plate 1 as uniform as possible, a distance l.sub.1 between the light source 6 and the plate 1 and a distance l.sub.2 between the light source 6 and the car body 2 should be made large. If the distance l.sub.2 is made too large, however, there is an aesthetics problem in that the light body 5 projects too far from the surface of the car body 2. Also, the upper portion of the number plate 1, which is the nearest to the light source 6, is more brightly lit than the lower portion, making difficult to read the number.
Particularly, as illustrated in FIG. 2, as to the parallel light rays a, b, c and d advancing as tangent lines on outer and inner circumferential surfaces of a glass bulb 6A of the light source 6 and all the parallel light rays included between the parallel light rays a and b, and c and d among the parallel light rays reflected from the reflector 7, reflection and scattering occur more strongly at the portions of the glass bulb 6A indicated with hatching than at other portions. Hence, ineffective dark portions 12 are produced, which appear as two shadow stripes on the surface of the plate.
Such a problem is present not only in a number plate lamp of the type in which the number plate 1 is illuminated from a position above the plate, but also in a number plate lamp of the type in which the number plate is illuminated from the left and right sides of the plate.